Loop antenna comprising plural helical coils on closed magnetic core



M. F. SPEARS 3,495,264 A COMPRISING PLURAL HELICAL N CLOSED MAGNETICCORE LOOP ANTENN COILS 0 Feb. 10, 1970 4 Sheets-Sheet 1 Filed Dec. 9,1966 kll l l Feb. 10, 1970 M F. SPEARS 3,495,264

LOOP ANTENNA COMPRISING PLURAL HELICAL COILS ON CLOSED MAGNETIC COREFiled Dec. 9, 1966 4 Sheets-Sheet 2 M 02 m PUT MATCH/N6 P fimcu I T rUNIT A HAS/N6- PROTECTOR F/l r51? NUWWK POWEF SUPPLY? Feb. 10, 1970 M.F. SPEARS 3,495,264 LOOP ANTENNA COMPRISING PLURAL HELICAL COILS ONCLOSED MAGNETIC CORE Filed Dec. 9, 1966 4 Sheets-Sheet 5 UNA Feb. 10,1970 M. F SPEARS LOOP ANTENNA COMPRISING PLURAL HELICAL COILS ON CLOSEDMAGNETIC CORE Filed Dec. 9, 1966 4 Sheets-Sheet 4 United States Patent3,495,264 LOOP ANTENNA COMPRISING PLURAL HELICAL COILS ON CLOSEDMAGNETIC CORE Morton F. Spears, Westwood, Mass., assignor, by mesneassignments, to Continental Electronics Manufacturing Company, Dallas,Tex., a corporation of Texas Filed Dec. 9, 1966, Ser. No. 600,466 Int.Cl. H01q 7/08 US. Cl. 343-788 7 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION Field of the invention The invention relatesto antennas which include magnetic material and are of the loop type.

Description of the prior art In designing antennas of this general type,several considerations must be recognized.

First, the configuration, weight, size and amount of the necessary fluxconducting material are important, because these largely determine thecost of the antenna and its utility in a given special environment.

Second, the electrical characteristics of the antenna must beconsidered. Generally, the greater the output for a given impedance, thebetter the antenna is. A greater output furnishes a greatersignal-to-noise ratio in circuits that follow the antenna. The outputcan be increased or decreased by simply adding or taking away turns fromthe windings, or by tuning the antenna to resonance or detuning it. Bothof these operations produce accompanying changes in impedance whichoffset any changes in the basic sensitivity. The sensitivity isdetermined by the ratio of the signal-to-noise voltages, and the noiseis proportional to the square root of the resistance component in theantenna. Therefore, at a given frequency and inductance, the qualityfactor (Q) which is determined by the ratio of inductance to resistanceshould be made as high as possible to reduce the resistance component. Alimit to Q, however, occurs for tuned circuits, when the informationfrequency bandwidth is reduced below that which will pass theinformation desired.

Finally, for an antenna of given impedance, there must be considered theoutput capability of the antenna, which may be characterized by itseffective height. For any antenna, the expression V =h E (where V is theoutput voltage of the antenna, B is the electric field strength of thetransmitted signal, and h is the effective height of the antenna) is ameasure of the antennas efficiency. Heretofore, in ferrite coreantennas, it has been possible to gain increases in effective heightonly at the expense of comparable increases in weight, inductance andnoise, or with reductions in bandwidth.

SUMMARY OF THE INVENTION Antennas according to the invention have a coreforming a closed path of flux conducting material and a pair of coilswound one each on a section of the closed core 3,495,264 Patented Feb.10, 1970 and connected in series. characteristically, the coils arewound in opposite directions, that is so oriented that the received,signal carrying, wave energy appears as magnetic flux of the samedirection for each core section, varying corresponding to signal andinducing additive signal output voltages across the series connectedcoils that are linked to the core sections. However, flux induced in thecore by the current in the coils is oppositely directed in the closedcore path, and cancels. Preferably the opposite sides of the core frameare parallel rods of any convenient cross-sectional shape, andpreferably each coil has the same number of turns, thereby providingbalanced cancellation. In further aspects of the invention, it ispreferable that the rods be substantially elongate to produce a higheffective height to antenna weight ratio, and it is preferable that eachcoil be wound substantially the entire length of the rod, to produce themaximum effective height.

In some practical applications, it is preferable to make the end pieces,or portions of the core which join the core sections such as the rodwhich carry the coils, higher than the thickness of the rods and widerthan the spacing between the rods, to increase the end area of the coreand to increase the elfective height of the antenna.

In another practical aspect, slotted conducting discs are placed overthe ends of the rods between the coils and the end pieces, in orderfurther to increase the effective height of the antenna.

A very useful practical embodiment of the invention has a rectangularcore, a first pair of coils wound one each on opposite sides of the corein opposite directions and connected in series, and a second pair ofcoils wound one each on the other two opposite sides of the core inopposite directions and connected in series, whereby the antenna canfunction as a direction finder or provide, with appropriate coupling,omniazimuthal reception.

In a still further aspect, a plurality of series connected oppositelywound coils can be mounted on opposite sections of the continuous core,whereby each coil unit will receive signals independently of the others,due to the induced flux cancellation principle inherent in theinvention.

In this manner, there are provided a higher antenna output for any giveninductance and a greater effective height per unit volume of fluxconducting material used than heretofore possible. These antennas aresimply and inexpensively constructed and can be adapted for variousspecial purposes such as direction finding, omniazimuthal reception, andindependent multiple signal reception; they are compact and suited forinstallation in confined environments and durable and reliable in use.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axonometric view of oneembodiment of the invention;

FIG. 2 is a similar view of a second embodiment of the invention;

FIG. 3 is a vertical section such as on plane 3-3 of FIG. 2, showing amodification of the invention;

FIG. 4 is a view of a composite antenna incorporating the invention;

FIG. 5 is a view of another embodiment of the invention and a circuitfor utilizing the output signal thereof;

FIG. 6 is a view of a further embodiment of the invention;

FIG. 7 is a top view of mounting structure for embodiments such as ofFIG. 5, of the antenna according to the present invention;

FIG. 8 is a section on line 8-8 of FIG. 7;

FIGS. 9, and 11 are views of shielding means used in the mountingstructure of FIGS. 7 and 8;

FIG. 12 is a sectional view of a mounting structure especially suitedfor embodiments of the invention such as according to FIG. 1, but with acore according to FIG. 4;

FIG. 13 is a partial section on line 13-13 of FIG. 12;

FIG. 14 is a partial section on line 14--14 of FIG. 12; and

FIGS. 15 and 16 are views of shielding means used in the mountingstructure of FIGS. 12 to 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The antenna according to FIG. 1has a core 1 forming a closed frame of magnetic flux conductingmaterial. This core has four sides here consisting of two substantiallyparallel rods 2 and 3 joined at their ends by flux conducting end pieces4 and 5. As described below with reference to other embodiments, thecore structure can be mechanically different, such as composed of barsof uniform cross section and equal length. On opposite sides of the core1, coils 6 and 7 preferably having equal numbers of turns are wound onrod 2 and on rod 3, respectively. These coils constitute antenna loops.

The coils 6 and 7 are wound in opposite directions on the rods 2 and 3and are connected in series between output terminals a and b'. Wound inopposite directions here means that looking at the series connectedcoils along their axes such as from the left of FIG. 1, and taking as astarting point for determining the winding direction of each coil theouter terminal associated with a respective coil, one coil windsclockwise from its outer terminal and the other coil windscounterclockwise from its outer terminal, each towards its inner seriesconnection. Thus, looking from the left of FIG. 1, coil 6 windsclockwise around rod 2 from terminal a towards the far end of the rodwith connection s and coil 7 winds conuterclockwise around rod 3 fromterminal b also towards s at the far end of the rod. Another example ofcoils wound in opposite directions is incorporated in the antenna ofFIG. 2. In this figure, again looking at the coils from the left of FIG.2, coil 16 proceeds clockwise from terminal 0 towards the right andseries connection t1 and coil 17 proceeds counterclockwise from terminald towards the left and t2.

Referring to FIG. 1, the antenna is shown disposed in a wave energyfield carrying the signal to be received, indicated by the arrows -H.The correspondingly varying magnetic flux H2, H3 appearing according towell known principles in the rods 2 and 3 of the core will inducevoltages across the series coils which will additively appear at outputterminals a, b. The output, then, is equal to twice the output of asingle rod antenna with a single coil. The voltage induced across theterminals a and b with current flow indicated by arrows on the coils,will induce in the core sections single flux H21, H31 components whichtend to oppose the flux caused in the respective sections by the signalH. Because the coils 6 and 7 are wound in opposite directions on thecore as explained above, this induced flux will appear in oppositedirections in the core sections and cancel to the extent the coils areeffectively coupled by the core. This cancellation is a maximum when thenumber of turns is the same on both sections. The inductance appearingat output terminals 0, b of the antenna is correspondingly reduced bythis cancellation, to a value far below that of two single rod antennaswith coils connected in series. It should be observed that while theinductance of the antenna is thus reduced, the voltage across theterminals a, b and the effective height of the antenna remainssubstantially equivalent to twice the voltage produced by a single rodantenna. In effect then, by winding the coils oppositely and linkingthem with a closed core of fluxconducting material as shown, it ispossible to obtain a virtual height twice the effective height of asingle rod antenna While substantially reducing the inductance of twosingle rod antennas in series. With inductance reduced at constant Q, abetter signal-to-noise ratio in the antenna itself is obtained, andgreater output is achieved relative to the noise of circuits followingthe antenna.

It will be evident that the above explanation with reference to FIG. 1applies analogously to FIGS. 2, 4, 5 and 6.

The amount of inductance reduction depends on the amount of couplingthat occurs in the core 1. Thus highly permeable substances make thebest materials for the core. Commercially available ferrite having atoroidal permeability [1. of approximately 2400 has been used withexcellent results, although materials of lower or higher permeabilitycan obviously be' used if desired.

Several refinements of the basic invention have been found to contributeto enhanced results.

It has been found that increasing the length to diameter ratio of therods 2 and 3 increases the effective height to antenna weight ratio.Thus, in other words, elongate rods produce a greater effective heightfor a given amount of ferrite than short rods. Where they can be used,long rods will therefore be less expensive for a given effective height.Such an antenna is shown in FIGS. 13 to 16.

A further refinement which increases effective height concerns thewinding of the coils 6 and 7. It has been found that maximum effectiveheight is obtained when the coils are wound on substantially the entirelength of the rods 2 and 3.

It has also been found that by providing enlarged end pieces 14 and 15,as shown in FIG. 2, the effective height of the antenna can besubstantially increased with a less than proportional increase ininductance. As shown, the end pieces 14 and 15 are each of height 71greater than the thickness t of the rods, and of width w greater thanthe outside spacing s of the rods 16 and 17.

A refinement which has been found useful in eliminating unwanted aircoupling or leakage of flux is shown in FIG. 3. Slotted conducting discs8 of aluminum for example, placed over one or more ends of the rods 2and 3 'between the end pieces 14 and 15 and the coils 6 and 7 to act asshorted loops, have helped to reduce the amount of stray coupling thattakes place, thereby further reducing the inductance of the antennawithout appreciable effect on the signal flux and hence Withoutappreciably reducing its effective height.

FIG. 4 shows a composite antenna utilizing the present invention. Thisantenna is enclosed in a streamlined housing of any suitableconstruction represented symbolically by a dotted line 20. One componentof the antenna is a balanced antenna 21 as heretofore described withreference to FIG. 1, having the form of a frame with long bars and shortend pieces, bars and end pieces being of similar cross sections. To pickup transverse signals and thereby provide omniazimuthal reception, asimple loop antenna coil 22 is Wound as a second component over theantenna 21 and over a plurality of toroidal ferrite frames such as rings23 disposed around the antenna 21. The output terminals are indicated atf, g for antenna 21, and at u, v for antenna 22. The long, narrowantenna 21 is well suited for placement in the longitudinal direction ofthe shell 20 and it has been found to work well in conjunction with theloop antenna 22 disposed with its cores in the shorter transversedirection and of equal length.

In a practical embodiment, this composite antenna was 21 inches longwith all core cross sections one inch square, with one-half inchseparation between the long rods, and with a diameter of the ferritetoroids 23 of about 6 inches. The balanced antenna component had 2 x 44turns, and the loop antenna component 27 turns, both of 36s litz wire.As a core material, Stackpole 24, with a m rating of 2400 wassatisfactory.

FIG. 5 shows another embodiment of the invention. This antenna has fourbars 31, 32, 33, 34 joined to form a rectangular, here square, core 30forming a closed frame of flux-conducting material. On two oppositerods, such as 31, 33, a first pair of coils 35, 36 are wound in oppositedirections and connected in series with output terminals A, B.Similarly, on the other two opposite rods 32, 34, a second pair of coils37, 38 are wound in opposite directions and connected in series withoutput terminals C, D. An electrostatic shield indicated schematicallyat 39 and preferably combined with a housing as shown in FIG. 7 enclosesthe antenna.

The antenna of FIG. 5 incorporates in essence two identical antennas ofthe type described with reference to FIG. 1, both of which use the samecore for induced flux cancellation, and which can be used individually,or, with appropriate phase additive circuitry, together constitute acrossed loop antenna for omniazimuthal reception. In accordance with theabove explained principle, flux induced in the core by current in anyone pair of series coils will have no net effect on the other pair, andthus the signals appearing at the two output terminal pairs A, B, and C,D will be entirely independent of each other. This aspect of the antennaof FIG. 5 renders it highly useful in direction-finding applications andin systems such as described in copending application Ser. No. 525,970,filed Feb. 8, 1966 now Patent No. 3,369,235. For example, the outputs ofthe orthogonal coils can be fed to two orthogonal fixed windings of aresolver r goniometer. The output is then a double figure eight patternwhich can be rotated in azimuth by simply turning the resolver rotor,which can be kept in proper azimuth relation to the transmitting stationeither manually or by automatic tracking from a gyro compass system.

The circuitry shown schematically in FIG. illustrates another way inwhich the two antenna outputs of this embodiment can be coupled, namelyfor utilization in omniazimuthal Loran-A reception.

For a given carrier signal of the form k sin wt, received from anyazimuth angle relative to the positioning of one pair of coils such as35, 36, the output signal at terminals A, B will be of the form k sin wtcos 1; and the output signal at terminals C, D will be of the form k sinwt sin g5, where k and k are arbitarary constants.

These antenna output signals are led, preferably by shielded twistedcable, to the coupler circuit shown in FIG. 5. The two antenna signalsgo first to input circuit protectors 50, 51 which protect furthercircuitry from high spurious signals, and then through matching unitsand filters 52, 53 to feedback preamplifiers 54, 55. The two amplifiedantenna outputs go from these preamplifiers to coupled phasing networks56, 57 which introduce a 90 degree phase difference between the signals.The output signals from the phasing networks, which are then of the formk cos wt cos and k sin wt sin 5, respectively, are summed at 58 toproduce a signal of the form k cos wt cos +k sin wt sin =k cos (wt). Thesummed signal is then amplified by power amplifier 59 for use in Loran-Aapparatus of known construction. A single power supply and regulator 60supplies the three amplifiers 54, 55 and 59.

The output from the antenna and circiut of FIG. 5 is of the form k cos(wt-) and it is thus apparent that a carrier phase alteration directlyrelated to the direction of reception is introduced. For systems such asLoran-A, which compares the time of arrival of pulse envelopes fromvarious directions, a shift in the carrier phase is unimportant, andtherefore the antenna of FIG. 5 can be used to advantage. Its low weightand flat configuration (compare FIGS. 7 to 11) enable it to be mountedwithout protruding and thereby causing drag, on aircraft and the like.Its low impedance and noise enable it to be matched into an amplifierwith little increase in noise so that limiting noise in a practical caseis primarily determined not by following amplifiers, but by the thermalnoise of the antenna alone. A high Q antenna can be used, therefore,even though the bandwidth desired is relatively broad, and broadbandatmospheric noise is the final limitation of sensitivity rather than theantenna.

FIG. -6 illustrates a further aspect of the present invention. In thisembodiment a core 70 forming a frame of flux conducting material hasparallel rods 71, 72 upon which a plurality of series connectedoppositely-wound coil pairs 73, 74, 75 are wound in opposite directionsas above explained. The current in any one coil pair produces oppositelydirected fluxes which cancel; it therefore follows that the current inany coil pair has no net effect on any other coil pair and the outputterminals of a coil pair can be loaded, shorted, or energized withoutaffecting the operation of the core 70 for other signals. Thus aplurality of coil pairs can be used as shown and multiple frequenciesfor multiple receivers can be utilized on the same core, with theadvantages that will be evident to those skilled in this art.

While the above-described embodiments incorporate rectangular coreframes which can be easily fabricated, it should be understood thattoroidal core shapes of any desirable configuration can be used ifdesired. For example, an elongate frame with rounded ends may be used toadvantage for a streamlined structure similar to FIG. 4. Also, polygonalsuch as triangular core frames can be used, with sharp or roundedcorners.

FIGS. 7 to 11 illustrate the previously mentioned practical mountingstructure for the square core antenna shown in FIG. 5.

The core 34 is held between two foam sheets 80, 81 which have chamferededges to receive and hold the inner portion of the core and whichtogether completely fill the open center of the core. Fastened to thesheets 80, 81 by bolts 82 are printed circuit board electrostaticshields 83 and 84, which overhang beyond the core. These shields, asshown in FIGS. 9 and 10, have strips or copper 83.1, 84.1 spaced bystrips of board. Strips 83.1 are interconnected as shown at 83.2. Thecopper strips of the boards are connected by copper tapes 85 as shown inFIGS. 9, l0 and 11. Cemented to one of the shield boards, such as 83, isa plurality of sockets 86 for fastening the antenna assembly to itssupport. Also cemented to the board 83 are conduit connectors 87, 88,for leads from the two pairs of coils of the antenna. Plastic material,such as solid polyurethane, is then applied by casting around thefastening means 86 and connectors 87, 88 to form a plate 89 on top ofand coterrninous with the board 83. The same plastic is cast into thespace outside the sheets 80, 81 and between the boards 83, 84 to form asecure collar 91 around the core 34. The antenna is supported virtuallyvibration-free by this mounting structure.

FIGS. 12 to 16 illustrate a practical structure for mounting rectangularantennas such as according to FIGS. 1 and 2.

Fastened to a base plate 101, on top of which is a bottom printedcircuit board shield 102 (FIG. 16), are two opposed L-shaped supportmembers 103.1, 103.2 which receive one end of the antenna core 105.Transverse to the L-shaped members are two opposed supports 106.1, 106.2which have tongues 107.1, 107.2 fitting into the open center of the core105 to hold it securely. The supports 106.1, 106.2 are fastened togetherby screws 108, 109 and have their bases resting on the shield board 102.These supports are made from syntheic dielectric material. A cylindricalprinted circuit shield 110 rests around the antenna with its bottom rimcemented to the bottom shield board 102. The cylindrical shield 110consists of printed circuit elements 110.1 which encircle the board ofthe shield proper, are joined by a spine 110.2, and open atdiametrically opposite points (not shown). The shield elements areproperly interconnected and if desired grounded. Using the cylindricalshield board 110 as a mold, a foam forming liquid is poured around theantenna to fill the cylinder completely. A top printed circuit boardshield 111 (FIG. 15) is placed at the top of the cylindrical shield 110.All

for the purpose of illustration only and that this invention includesall modifications and equivalents which fall within the scope of theappended claims.

I claim:

1. An antenna comprising:

a core forming a closed path of flux conducting material having twoparallel, high permeability flux-conducting rods upon which the coilsare wound, and two enlarged likewise high permeability flux-conductingend pieces for connecting the adjacent ends of the rods; and

a pair of coils wound on opposite sections of the core and connected inseries, the coils being wound in opposite directions on their respectivesections, so that signal-carrying wave energy appears within the coresections as correspondingly varying flux of one direction inducingadditive voltages across the series coils, whereas the opposite fluxinduced in the core sections by the currents in the respectiveoppositely wound series coils cancels;

said enlarged end pieces being of essentially similar magnetic materialas the rods, of height greater than the thickness of a rod, and of widthgreater than the outside spacing of the rods.

2. An antenna comprising:

a core forming a closed path of flux conducting material having twoparallel fiux-conducting rods upon which the coils are wound and fluxconducting means for connecting the adjacent ends of the rods;

a pair of coils wound on opposite sections of the core and connected inseries, the coils being wound in opposite directions on their respectivesections, so that signal-carrying wave energy appears within the coresections as correspondingly varying flux of one direction inducingadditive voltages across the series coils, whereas the opposite fluxinduced in the core sections by the currents in the respectiveoppositely wound series coils cancels; and

slotted disc means placed at the end of a rod between the coil and theend piece, for reducing air coupling of flux.

3. An antenna comprising:

a core forming a closed path of flux conducting ma terial;

a pair of coils wound on opposite sections of the core and connected inseries, the coils being wound in opposite directions on their respectivesections, so that signal-carrying wave energy appears Within the coresections are correspondingly varying flux of one direction inducingadditive voltages across the series coils, whereas the opposite fluxinduced in the core sections by the currents in the respectiveoppositely wound series coils cancels;

a plurality of flux conducting frames disposed around the core; and

a loop coil wound over the frames and the core with it axis intersectingthe direction of the axes of the pair of coils on the core.

4. Antenna according to-claim 3 wherein:

the core sections are elongate and connected by comparatively short endpieces,

the frames are essentially annular, and

the loop coil is wound over the end pieces and over corresponding frameportions.

5. An antenna comprising:

a core forming a closed path of fiux conducting material; and

a plurality of pairs of coils similarly wound on each one of oppositesections of the core and connected in series, the coils of each pairbeing wound in opposite directions on their respective sections, so thatsignal-carrying wave energy appears within the core sections ascorrespondingly varying flux of one direction inducing additive voltagesacross the series coils, whereas the opposite flux induced in the coresections by the currents in the respective oppositely wound series coilscancels each of the pairs of coils constituting an individual antennawith individual output terminals.

6. A composite antenna comprising:

a core having a first and second pair of rods forming opposite sides ofa closed frame of flux conducting material;

a first pair of coils one coil wound on each rod of the first pair ofrods in opposite directions and connected in series; and

a second pair of coils one coil wound on each rod of the second pair ofrods in opposite directions and connected in series,

whereby signal carrying wave energy appears within respective pairs ofopposite rods as correspondingly varying flux of one direction inducingadditive voltages across the series coils on said pairs of rods, whereasthe opposite flux induced in the core sections by the currents in therespective oppositely Wound series coils cancels, and whereby thecomposite antenna constitutes two individual antennas which can be usedseparately or as orthogonal components for omniazimuthal reception.

7. Antenna according to claim 6 wherein the pairs of rods are of equallength and at right angles to each other, the coils of identicalelectrical dimensions, and the coil terminals are connected to form acrossed loop system.

References Cited UNITED STATES PATENTS 2,375,593 5/1945 Sontheimeretal.343--788 2,955,286 10/1960. Klein 343-788 OTHER REFERENCES Germanprinted application, K 22,888, July 19, 1954,

Kurt Kaschke.

ELI LIEBERMAN, Primary Examiner US. Cl. X.R. 343-842, 867

